Gas-fired domestic ovens

ABSTRACT

In a domestic gas-fired oven of the pyrolytic self-cleaning type with a forced-draught fan re-circulating a portion of the combustion products back through the oven during self-cleaning the re-circulation ratio, that is to say, the ratio of the total mass circulating to the net mass entering and leaving the system, is between three to one and five to one, being preferably four to one.

United States Patent Toth et al.

[54] GAS-FIRED DOMESTIC OVENS [72] Inventors: Bela M. Toth; Ian Thow,both of Solihull,

England Parkinson Cowan Limited, London, England [22] Filed: June19,1970

[21] App]. No.: 47,798

[73] Assignee:

| '11)] Foreign Application Priority Data June 2|, I969 Great Britain..3l,45l/69 521 u.s.c|. ..l26/21A 1 [58] Field ofSearch ..l26/21,2l A,19,39,273

[56] References Cited UNITED STATES PATENTS 3,437,085 4/1969 Perry126/21 A [451 June 6,1972

3,384,068 5/1968 Perry et al l 26/21 A FOREIGN PATENTS OR APPLICATIONS742,248 l2/l955 Great Britain ..l26/2I A 1,142,804 H1963 Germany I 26/21A Primary Examiner-Charles J. Myhre Attorney-Scrivener. Parker,Scrivener & Clarke ABSTRACT In a domestic gas-fired oven of thepyrolytic self-cleaning type with a forced-draught fan roe-circulating aportion of the combustion products back through the oven duringself-cleaning the re-circulation ratio, that is to say, the ratio of thetotal mass circulating to the net mass entering and leaving the system,is between three to one and five to one, being preferably four to one.

3 Claims, 2 Drawing Figures GAS-FIRED DOMESTIC OVENS This inventionrelates to self-cleaning gas-fired domestic ovens, that is to say, ovensin which it is possible to raise the temperature of the oven wallssubstantially above the normal range of cooking temperatures to atemperature level at which any particles of grease or food adhering tothe oven walls are burner and also for re-circulating the gases withinthe oven.

Other proposals have involved the provision of an additional burner orburners to be brought into action for the self-cleanin g function. I

A problem with such arrangements is that of controlling the temperaturewhich the oven is to reach during self-cleaning and this is normallydealt with by the provision of a high-temperature thermostat, whichswitches the burner, or one of the burners, on and off in a cyclicmanner to maintain the temperature within the desired range. However,such thermostats capable of operating at a high temperature, well abovethe normal cooking temperature range, are expensive and they addappreciably to the cost of the system.

A further problem in self-cleaning gas-fired ovens lies in the largeheat output which must somehow be disposed of without overheating thekitchen or other room in which the cooker is situated, and withoutdamaging its paintwork or giving rise to the danger offire.

The aim of the present invention is to provide a self-cleaning systemwhich overcomes these problems by arranging the conditions of operationduring self-cleaning such that the desired temperature is reached withthe minimum of control equipment and with the minimum of heat loss tothe surroundings.

According to the invention we now propose an oven-cleaning systemcomprising a single forced-draught ventilation system arranged tore-circulate the gases within the oven with a re-circulation ratio (asdefined below) lying between three to one and five to one, preferablysubstantially four to one.

In this way it is possible to obtain effective self-cleaning with only asinge burner, preferably mounted at the bottom of the oven, and thechoice of the re-circulation ratio lying within the limits set out abovemakes it possible to attain equilibrium conditions in the oven at apre-arranged temperature level, thereby eliminating the need forthermostatic control; it is only advisable for safety reasons to providea cheap form of thermal cut-out to shut down the burner in the event ofa fault giving rise to dangerous temperature levels.

The re-circulation within the limits defined allows the requiredtemperature to be attained with a burner of only modest size and it hasthe added advantage of reducing the heat escaping to the surroundingsduring self-cleaning.

The re-circulation ratio, referred to above, is defined as the ratio ofthe total mass flow of gases through the oven at equilibrium conditionsto the total mass input of air and gas.

Preferably the gases for re-circulation and the gases to be dischargedto the atmosphere are extracted from the oven through a common flue,placed low down in the rear wall of the oven.

The invention will now be further described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic vertical cross-section through an ovenaccording to the invention, looking from one side; and

FIG. 2 is an isometric view of the burner and re-circulating fanassembly.

Referring first to FIG. 1, the oven 1 can be of orthodox constructionexcept that it is much more heavily lagged against heat loss than anormal non-self-cleaning oven. A trough 2 near the rear edge of thefloor of the oven contains an elongated burner 3 having, in the exampleshown, a maximum heat output of 13,000 B.Th.U. (3,250 Kilocals) perhour. Gas is supplied to the burner as indicated at G and primary air Pfor combustion is entrained by the usual mixing tube 4.

A fan 5 of the tangential type driven by an electric motor 6 is dividedinto two unequal sections 7 and 8, of which the smaller section 7 drawsin air S from the atmosphere to form secondary air for combustion. Theoutputs of both sections of the fan lead to a slot in the bottom of thetrough so that the output flows upwards past and around the burner, andassists combustion as well as causing a circulating flow of the productsof combustion around the oven as indicated by the arrow C.

Combustion products escape through an outlet 9 and are divided, a minorpart passing upwards as indicated by the arrow F through nozzles 10 toatmosphere, where they entrain outside air, as indicated by arrows A,and then pass upwards, cooled and diluted by this air, to a flue 11.

The major part of the combustion products entering the outlet 9 is drawnthrough a duct 12 by the larger section 8 of the fan 5 as indicated bythe arrow R and re-circulated through the oven.

Thus the total gases circulating in the oven 1 are formed by the totalmass flow C of the gases represented by the arrows G, P, S and R. Thetotal mass entering the system is represented by G, P and S. The totalmass F leaving the system must necessarily be equal to this.

The re-circulation ratio is the ratio (G P S R) to (G P S). This isequivalent to the ratio (F R) to F. Thus if one quarter of thecombustion products entering the outlet 9 pass to the flue andthree-quarters are re-circulated through the duct 12, this means R isthree times F (which equals G P S) and the re-circulation ratio is four.

Tests have shown that with a typical domestic oven with a volume of 2.5cubic feet (71 liters), having the burner mentioned above, with a heatoutput of 13,000 B.Th.U (3,250 Kilocals) per hour, and consuming a totalof 15.5 lb. (7 Kg.) per hour of gas and air, while maintaining are-circulation R of 46.7 lb. (21.1 Kg.) of combustion products per hour(giving a re-circulation ratio of 4), the oven attains an equilibriumtemperature of 522 C. This is adequate for burning off food and greaseparticles and leaving the walls of the oven completely clean.

The primary air P is arranged to provide about 60 percent of the airrequired for complete combustion of the gas according to stoichiometricprinciples. The secondary air S is arranged not only to provide theremaining 40 percent for combustion of the gas but also a further 70percent excess air, making a total of 170 percent of the quantity neededfor burning the gas, and this excess air is present to oxidize and burnaway the waste products on the oven walls. I

With the equilibrium conditions mentioned above the amount of heat thatis recirculated through the duct 12, as opposed to the massre-circulated, is between 50 and 70 percent of the net heat output ofthe burner, i.e., between 6,000 and 9,000 B.Th.U. 1,500 and 2,250Kilocals) per hour where the burner output can vary between 1 1,500 and16,000 (2,875 and 4,000 Kilocals) per hour. With the burner giving13,000 B.Th.U per hour, 62 percent of the heat input is beingre-circulated (8,000 B.Th.U. or 2,000 Kilocals per hour).

It will be understood that the-equilibrium temperature attained can bevaried by varying the heat input from the burner. In practice it shouldbe at least 425 C. in order to ensure adequate cleaning of the ovenwalls and if it is allowed to rise above 650 C there is danger of damageto the enamel coating on these walls.

The value of four to one for the re-circulation ratio has been found toprovide the optimum balance between efficient combustion and minimumunnecessary heating of the surroundings, as well as achieving maximumeconomy. If the ratio is substantially increased, i.e. if a smallerproportion of the combustion products are allowed to escape, the flamestability is adversely affected by the high proportions of combustionproducts around the burner, incomplete combustion occurs and the systemis no longer able to continue the cleaning process. In practice there-circulation ratio can be increased a little above four, but cannot bemore than five to one without inefficiency.

Likewise, if the re-circulation ratio is reduced appreciably below fourto one resulting in a higher proportion of the combustion products beingallowed to escape to the atmosphere, not only does this increase theundesirable discharge of large quantities of hot products to be disposedof, but also it results in a reduction in oven temperature due todilution of the cir culating gases with cold incoming air. In practiceworthwhile results are not obtainable with a ratio of less than three toone.

It will be understood that for normal operation of the oven for cookingor baking, the same burner can be used, but at a reduced heat output.The fan 5 can be kept running to circulate the combustion productsand-ensure even temperature conditions in the oven. The burner may be ofthe duplex type, that is to say, of the type in which the gas flow toone group of the ports is controlled separately from that to anothergroup, and one group may be cut off altogether during cooking. Underthese conditions the oven temperature can be controlled by thenormaloven thermostat acting on the burner.

As indicated above, the temperature during self-cleaning is set solelyby the heat balance attained under equilibrium conditions and nohigh-temperature thermostat is used. However, there is preferably asimple thermal cut-out responding to severe excess temperatureconditions, for reasons of safety.

In FIG. 2 is shown a practical layout for the burner and fan. The samereference numerals as in FIG. 1 have been used where applicable The ovenitself has been omitted. Two outlets from the oven, shown at 9 and 9"enter a cylindrical manifold 13 from which a minor part of thecombustion products escape through nozzles 10 to the flue 11, while there-circulation duct 12 leads off at right angles from the middle .Of themanifold. The fan 5 is secured to the bottom of the burner trough 2. Itwill be noted, as indicated by the arrow A, that the fan rotates in theopposite direction to one which one would expect, and so its output isacross rather than along its tangential output duct that leads into thetrough 2. This helps to ensure good turbulence and mixing of theincoming secondary air and the re-circulated products as they both reachthe burner.

It will be understood that the relative values of the different massesflowing can be controlled by appropriately selecting the relativecross-sections of the oven outlets 9 and 9", the nozzles 10, and thesections 7 and 8 of the fan. The inlet to the section 7 is preferablymade easily adjustable to control the quantity of secondary air.

In the preferred embodiment the outlets 9' and 9" are 1% inches abovethe floor of the oven; the nozzles 10 have a total cross-sectional areaof not more than 5 square inches. The cross-sectional area, measured ina horizontal plane, of the trough 2 is up to thirty square inches, withabout half of this cross-section occupied by the burner 3.

In a modification it may be possible to mount the burner in the top ofthe oven instead of the bottom, in which case the overall system couldbe similar to that illustrated in FIGS. 1 and 2 but inverted.

We claim:

1. A self-cleaning gas-fired domestic oven comprising an oven wallstructure defining an oven cavity, a gas-fired burner disposed-in saidwall structure and serving to heat said cavity, a gas inlet pipesupplying said burner, structure defining an air inlet opening to theneighborhood of said burner, structure defining a first exhaust openingfor heated air and combustion products from said oven cavity, structuredefining a second exhaust opening for heated air and combustion productsfrom said oven cavity, a recirculation duct having an entry end and anexit end, said entry end communicating with said second exhaust opening,whilst said first exhaust opening communicates with atmosphere, the exitend of said duct communicating with said oven cavity in theneighbourhood of said burner, and a power-driven fan disposed in saidduct, the dimensions and power of said fan and the dimensions of saidduct and second exhaust opening being selected in relation to thedimensions of said first exhaust opening and of said gas inlet pipe andair inlet openin that under equilibrium conditrons and with said burnerin ull operation the total mass of gases entering said cavity in theneighbourhood of said burner from said duct, said air inlet opening andsaid gas inlet pipe lies between three times and five times the mass ofgas escaping to atmosphere through said first exhaust opening.

2. The oven set forth in claim 1 wherein said dimensions are such thatsaid total mass of gases entering said cavity is substantially fourtimes the mass of gases escaping to atmosphere through said firstexhaust opening. I

3, The oven set forth in claim 1 including a trough disposed in thebottom of said wall structure, said burner being disposed in said troughand said duct exit end opening into the bottom of said trough.

1. A self-cleaning gas-fired domestic oven comprising an oven wallstructure defining an oven cavity, a gas-fired burner disposed in saidwall structure and serving to heat said cavity, a gas inlet pipesupplying said burner, structure defining an air inlet opening to theneighborhood of said burner, structure defining a first exhaust openingfor heated air and combustion products from said oven cavity, structuredefining a second exhaust opening for heated air and combustion productsfrom said oven cavity, a recirculation duct having an entry end and anexit end, said entry end communicating with said second exhaust opening,whilst said first exhaust opening communicates with atmosphere, the exitend of said duct communicating with said oven cavity in theneighbourhood of said burner, and a powerdriven fan disposed in saidduct, the dimensions and power of said fan and the dimensions of saidduct and second exhaust opening being selected in relation to thedimensions of said first exhaust opening and of said gas inlet pipe andair inlet opening that under equilibrium conditions and with said burnerin full operation the total mass of gases entering said cavity in theneighbourhood of said burner from said duct, said air inlet opening andsaid gas inlet pipe lies between three times and five times the mass ofgas escaping to atmosphere through said first exhaust opening.
 2. Theoven set forth in claim 1 wherein said dimensions are such that saidtotal mass of gases entering said cavity is substantially four times themass of gases escaping to atmosphere through said first exhaust opening.3. The oven set forth in claim 1 including a trough disposed in thebottom of said wall structure, said burner being disposed in said troughand said duct exit end opening into the bottom of said trough.